Products with low Inter-Modulation Distortion (IMD) characteristics are becoming increasingly important factor in wireless communication links. In cellular communication base stations for example, two or more carriers may share the same antennas, amplifiers, filters, etc., where non-linearity in the components may cause mixing of the carriers to produce unwanted spurious signals that interfere with the operation of the system. Inter-modulation caused from passive components, i.e., Passive Inter-modulation (PIM), is one problematic source of IMD in communication systems. Previously known PIM sources include diodes, ferromagnetic materials in conductors and connectors, junctions between galvanically dissimilar metals, and junctions between metals and oxides that exhibit diode-like behavior.
An example of a 0.5 W cell phone communicating with a base-station illustrates the significance PIM can have in cellular communications. The signal from the cell phone may be as low as −110 dBm when received. Often a two-tone model is used for the carrier frequencies, with each tone at 20 W, or +43 dBm. This amounts to extracting the cell phone signal at −153 dBc below the carrier frequencies. Thus, any PIM noise caused by the carriers must be below a −153 dBm noise floor to avoid adverse effects on the system.
In filter design, the PIM requirements may be even more extreme. To guarantee that base-stations are deployed to comply with the stringiest PIM specifications, special filter solutions are needed for use in the components of the base-station, as well as in the test equipment used to develop, install, and calibrate those components. For example, in test set filters used for broad emission monitoring, an additional −3 dBm of margin resulting in a −156 dBm PIM requirement may be needed to guarantee that the test filters are not injecting noise into a device under test (DUT) that would provide misleading results.